In today's automotive market, there exists a variety of electric propulsion or drive technologies used to power vehicles. The technologies include electric traction motors such as DC motors, AC induction motors, switched reluctance motors, synchronous reluctance motors, brushless DC motors, permanent magnet synchronous motors (PMSM) and corresponding power electronics. PMSM motors are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to regular DC motors and AC induction motors. PMSM motors typically operate with a permanent magnet rotor. A permanent magnet rotor may be configured as a surface mount or interior or buried permanent magnet rotor. An interior permanent magnet (IPM) motor or machine has performance attributes, when compared to regular DC motors and AC induction motors, that include relatively high efficiency, relatively high torque, relatively high power densities, and a long constant power operating range which make an IPM motor attractive for vehicle propulsion applications.
Permanent magnets buried inside a rotor of a PMSM motor exhibit high reluctance directly along the magnetic axis or the d-axis due to the low permeability of the permanent magnets. While along the q-axis, between the magnetic poles or magnet barriers of an IPM rotor, there exists no magnetic barrier, and reluctivity to magnetic flux is very low. This variation of the reluctance around the rotor creates saliency in the rotor structure of an IPM machine. Therefore, the IPM rotors have reluctance torque in addition to the permanent magnet torque generated by the magnets buried inside the rotor.
The magnets in the motor are arranged in several layers creating a multi-barrier design. The multi-barrier design reduces leakage and improves the rotor saliency. Accordingly, motors having multi-barrier rotors have numerous performance advantages over a single barrier rotor designs, including relatively high overall efficiency, extended high-speed constant power operating range, and improved power factor. Reducing dependency on magnetic torque helps lower the number of magnets or amount of magnetic material in an IPM machine, as compared to a single barrier IPM machine or surface mounted permanent magnet machine. The amount of magnetic material needed to generate a specific torque and wattage rating depends on the level of saliency of the rotor. The higher the rotor saliency, the lower the magnet material usage for the same overall machine performance. Electric motors having a multi-barrier rotor design, as compared to single barrier design, generate higher rotor saliency.
The reduction of magnetic material in an electric motor rotor is desirable from a cost standpoint. Consequently, deep cavities are generally left empty as their contribution to the rotor magnetic field should they be filled is relatively small. The reason for this is the relative distance to the motor airgap. A pure synchronous reluctance motor that has similar rotor geometry to the multi-barrier permanent magnet (PM) design, but no magnetic material in the rotor, is a relatively low performance machine. Multi-barrier IPM electric motors have the beneficial attributes of both synchronous reluctance machines and the permanent magnet machine and are therefore excellent candidates for vehicle propulsion. A major difficulty involved with IPM machines is the design and manufacture of the rotor.